t7502: properly quote GIT_EDITOR

One of the tests tries to ensure that editor is not run due to an early failure. However, it needs to quote the pathname of the trash directory used in $GIT_EDITOR, since git will pass it along to the shell. In other words, the test would pass whether the code was correct or not, since the unquoted editor specification would never run. We never noticed the problem because the code is indeed correct, so git-commit never even tried to run the editor. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>